Propagate linker args upwards in the dependency graph.

[jeff-davis]

Suggestion from @froydnj to enable a build script to pass linker args
relevant for dependencies of a crate.

The use case is to allow specifying "-Wl,-undefined,dynamic_lookup" on
MacOS for dynamic library crates with undefined symbols, such as a
symbol defined in the program loading the library.

We can consider supporting this case in rustc itself to avoid the need
for platform-specific flags in typical cdylib cases. But this seems
like an independently-useful mechanism that could enable workaround
for other linking problems, as well.

Enables workaround for #62874

[rust-highfive]

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[jeff-davis]

@alexcrichton @froydnj

[alexcrichton]

Thanks for the PR!

I would unfortunately though have similar concerns for this as I would for rust-lang/rust#66204. Linker arguments are really hard, and this is unfortunately likely to cause regressions by changing the behavior of link argument propagation.

I think to fix this we'd likely want to add a new directive for Cargo which specifically requests the transitive propagation of link arguments.

Additionally this will need tests. For example link arguments from proc macros should not be propagated.

[froydnj]

Alex or somebody else is the right person to handle this.

[jeff-davis]

Are you really sure this is the right direction? I feel like this is getting more complex and fraught; not less. Even if we make this cargo PR acceptable, it still leaves a potential trap for any developer that doesn't test on MacOS.

[bors]

☔ The latest upstream changes (presumably #8441) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

[ehuss]

Closing due to inactivity. Larger features like this probably should go through the RFC process to determine what the fundamental use cases and design considerations are.